docs/docs/routing/bird/maxlen-filter.md

# Implement max-len filter

To implement a max-length filter, we need a rule/policy/filter file that contains the corresponding prefixes with their maximum length and a list with the prefixes without maxlen to distinguish if the prefix belongs to CRXN or to dn42. The entitydb contains a bash script `build_maxlen_filter.sh` for this purpose. If you run this script with the argument `bird`, you will get a bird-compatible list with the maximum lengths. If you run the script with the `bird prefix-list` parameter, you will get a prefix list without the maximum lengths.

```
define CRXN_IPs = [
    include "<path-to-file-1>";
];

define CRXN_MAXLEN = [
    include "<path-to-file-2>";
];

function is_crxn_net() {
  return net ~ CRXN_IPs;
}

function is_maxlen_valid() {
  return net ~ CRXN_MAXLEN;
}
```
This bird configuration can be used to load the file. You have to replace `<path-to-file-1>` with the path of the filter file without maximum lengths and `<path-to-file-2>` with the path of the filter file with maximum lengths. Function `is_crxn_net` then checks if the prefix belongs to the CRXN network and function `is_maxlen_valid` then checks if the prefix is maxlen-valid.

```
if (is_crxn_net() && (! is_maxlen_valid())) then {
    print "[CRXN] Invalid crxn route: ", net;
    reject;
}
```
This instruction checks whether a prefix belongs to the CRXN network and then whether the prefix is maxlen-valid. If it is not maxlen-valid, it is filtered and a message is issued.

This configuration can then be added to your import filter before `accept`. The line with `print` is optional. If you leave this line, every hjack attempt will be logged in the bird log output.